Plutonium Signatures in a Dated Sediment Core as a Tool to Reveal Nuclear Sources in the Baltic Sea

Plutonium distribution was studied in an undisturbed sediment core sampled from the Tvären bay in the vicinity of the Studsvik nuclear facility in Sweden. The complete analysis, including minor isotopes, of the Pu isotope composition (238Pu, 239Pu, 240Pu, 241Pu, 242Pu, and 244Pu) allowed us to establish the Pu origin in this area of the Baltic Sea and to reconstruct the Studsvik aquatic release history. The results show highly enriched 239Pu, probably originating from the Swedish nuclear program in the 1960s and 1970s and the handling of high burn-up nuclear fuel in the later years. In addition, the 244Pu/239Pu atomic ratio for the global fallout period between 1958 and 1965 is suggested to be (7.94 ± 0.31)·10–5. In the bottom layer of the sediment, dated 1953–1957, we detected a higher average 244Pu/239Pu ratio of (1.51 ± 0.11)·10–4, indicating the possible impact of the first US thermonuclear tests (1952–1958).

• Section 1: Sample preparation and validation of the results • Supporting figures

Sample preparation and validation of the results
A first set of 46 samples (Group 1) were processed at SSM laboratories (Swedish Radiation Safety Authority, Stockholm, Sweden). For this, about 3 g of sediments from all the available layers were used, and Pu fractions from those samples were purified and electrodeposited on planchets for alpha spectrometry analysis (i.e. 238 Pu, 239+240 Pu) as it is described in methods section. 242 Pu was used as spike to quantify the final concentrations.
Using the information provided by alpha spectrometry (see fig. 2-a, 3-a and 3-b), 24 samples from the most relevant layers were selected to study isotopic ratios by AMS (i.e. 240 Pu/ 239 Pu, 241 Pu/ 239 Pu and 244 Pu/ 239 Pu). To this end, alpha planchets were leached to recover the electrodeposited Pu and, afterwards, produce the AMS cathodes 1 .
Finally, 29 layers from the core were chosen to produce new samples from the remaining original material in order to complement the Pu isotopic composition by analysing the 242 Pu by AMS (Group 2). About 4-5 grams of samples were used and no spike was added to those samples. Therefore, only information about the isotopic composition of the samples was evaluated. To calculate the corresponding concentrations, 239,240 Pu activity concentrations measured by alpha spectrometry from Group 1 were used. Those samples were processed at CNA laboratories (Centro Nacional de Aceleradores, Sevilla, Spain). In addition to 242  Duplicate samples showed very consisted results for the studied ratios (Fig. S1). Although a slight tendency of higher 244 Pu/ 239 Pu ratios from group 1 seems to be observed, the results agree within uncertainties in the most of the cases. In all the AMS measurements, similar background levels were obtained from the instrumental blanks (i.e. below 5 total background counts in a total measurement time of 30 min for the mass 244) and the reliability of the measured ratios and the stability of the AMS analysis was control by analysing several standard samples (i.e. ColPuS 2 ). Thereby, any evidence of a systematic offset during the analysis was found.
Due to the extremely low concentrations of 244 Pu in the samples the results were on the verge of the detection limit (i.e. 10 5 atoms), presenting high uncertainties in some cases (i.e. with a final relative uncertainty ranging from 8% to 72%) but only one sample was not possible to quantify (i.e. 1cm depth). Despite this, the reliability of those samples is supported by the good agreement between dataset from Group 1 and 2.
Final results were calculated as the weighted average considering the uncertainties in the cases of duplicate samples.

Materials and blanks
The first set of samples (Group 1) was prepared at SSM laboratories. Nitric acid (65%), hydrochloric acid (37%), ammonia (25%), ascorbic acid, hydroxylammonium hydrochloride, potassium disulfate, sodium hydroxide, sodium sulphate, thymol blue were obtained from Merck (Darmstadt, Germany). All used chemicals were of analytical grade. 242 Pu internal tracer was obtained from National Institute of Standards and Technology (Gaithersburg, MD, USA). Along with the samples two different blanks were processed: i) procedural blanks (i.e. processed blanks samples under the same conditions as S3 the samples of interest) and ii) electrodeposition blanks (i.e. spiked solutions directly electroplated together with the real samples to control the background of this specific step).
Samples from Group 2 were processed at the CNA laboratories and no spike was added to measure the 242 Pu in the samples. For those samples, being specifically intended for AMS and aimed for analysis of very small concentrations of 242 Pu and 244 Pu, special protocols to keep the laboratory contamination at a minimum were followed. Specifically, for the study of 242 Pu, the use of new materials to avoid contamination from the commonly used 242 Pu spike became critical. Additionally, acids of the highest purity were used in every stage, and for the AMS cathode preparation, a Fe(III) solution provided by High Purity Standards (HPS, England) was used, with a certified 238 U concentration below the 0.5 ppb level. The Nb powder added during this last step was provided by Sigma-Aldrich® (purity of 99.8%). Several procedural blanks were prepared together with those samples.
TEVA resin cartridges were obtained from Triskem International (Bruz, France). Ultra-pure water obtained from a Milli-Q system water purifier (Millipore Corp., Bedford, UK) was used for all the experiments.
Processed blanks from Group 1 and 2 presented similar backgrounds, both between them and compared with the non-processed blanks. Therefore, no additional background correction from the laboratory processing was necessary.

Alpha spectrometry quality control and quality assurance
The used plutonium separation method was controlled by using IAEA-135 marine sediment reference material. The obtained results were from 43.2±1.9 to 50.1±0.9 for 238 Pu, and from 216±3 to 240±3 Bq/kg for 239+240 Pu. The reference confidence interval (α=0.05) for analysed Pu isotopes given in the certificate is: 41.6 to 45 Bq/kg for 238 Pu and 205 to 225.8 Bq/kg for 239+240 Pu. The choice of IAEA-135 reference material was to ensure complete Np separation with 242 Pu tracer (IAEA-135 marine sediment contain alpha spectrometry measurable 237 Np activities).